how to display Horizon line on camera view image?
I am not at all sure if that's what you're searching, but if you want to display the middle of the camera, that's done in the camera settings. Just select it and go to the following place:



If however you need a custom guide like this:



or this:



...then there's at least 3 things you can do:


Bribe a dev or become one 
Do serious Python work and write an Addon
Make a mesh consisting of edges and place it inside the camera.


Options one and two are fascinating yet option 3 is fast and easy.



Just model whatever guide you need out of edges and place the object into your camera. Make sure it's visible from camera view. Put the guide's object origin at the camera origin to avoid relationship lines. Parent the object to the camera. And in the outliner, disable rendering and selection.



If you want colors, just use a grease pencil line instead. You can edit them similar to a mesh.



The stroke can be parented to the camera as well.



For rendering, I'd probably just create an image of the line with transparent background in Gimp or Blender and then composite it on top of the rest with the Node Editor and an Alpha Over node.



Images usually render faster than most scenes that recalculate stuff for every frame.